The problem is quite evident.
Bernardeschi has never had the football brain, passing, vision, or decision making to play as trequartista.
His best position is winger, and he used to be quite a good winger at Fiorentina, and in his first season for Juve.
But now he's gained weight and become fat, he can't play winger anymore because he is too slow for that position.
He's no longer a winger and was never a treq, so what is he now?